# LINQ 小知識 `IEquatable<T>`, `IEqualityComparer<T>`

## LINQ 的 `Distinct()`, `Except()`, `Intersect()`, `Union()` 沒有得到預想的結果

在下面一段Code中 LINQ To Object 用 `Enumerable.Distinct()` Method, `Enumerable.Except() Method`, `Enumerable.Union() Method` 為甚麼資料都沒有去除重複，取差集, 聯集 資料也沒有得到預想的結果

 /// <summary>
 /// Sets the engagement team members.
 /// </summary>
 private void SetEngagementTeamMembers()
 {
     var teamMemberEngagementNumberEqualityComparer = new EngagementTeamMemberEngagementNumberEqualityComparer();
     var engagementTeamMembers = GetEngagementTeamMember(teamMemberEngagementNumberEqualityComparer)
         .ToList();
     var engagementMemberByTimeSheet = GetEngagementMemberByTimeSheet(teamMemberEngagementNumberEqualityComparer)
         .ToList();
     var allTeamMembers = engagementTeamMembers
         .Union(engagementMemberByTimeSheet, teamMemberEngagementNumberEqualityComparer);
     var existedEngagementTeamMembers = GetExistedEngagementTeamMembers()
         .Distinct(teamMemberEngagementNumberEqualityComparer);
     var allNeedAddTeamMembers = allTeamMembers
         .Except(existedEngagementTeamMembers, teamMemberEngagementNumberEqualityComparer)
         .ToList();
     var teamMembersByTimeSheet = allNeedAddTeamMembers
         .Except(engagementTeamMembers, teamMemberEngagementNumberEqualityComparer)
         .ToList();
     var needAddTeamMembers = allNeedAddTeamMembers
         .Except(teamMembersByTimeSheet, teamMemberEngagementNumberEqualityComparer)
         .ToList();
     EngagementTeamMembers = needAddTeamMembers
         .GroupBy(c => c.EngagementNumber)
         .ToDictionary(c => c.Key, h => h.Distinct(teamMemberEngagementNumberEqualityComparer).ToList());
     EngagementTimeSheetUsers = teamMembersByTimeSheet
         .GroupBy(c => c.EngagementNumber)
         .ToDictionary(c => c.Key, h => h.Distinct(teamMemberEngagementNumberEqualityComparer).ToList());
 }

原因是在

`Distinct<TSource>(IEnumerable<TSource>)`
Returns distinct elements from a sequence by **using the default equality comparer to compare values**.

`Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>)`
Produces the set difference of two sequences by **using the default equality comparer to compare values**.

如果是比較`Object`, `Reference types` 的物件，預設是用 **memory addresses** 來做比較

以下的 Code `member1`, `member2` 是不相等的

class Program  
{  
    static void Main(String[] args) {  
        var member1 = new EngagementTeamMember();  
        member1.EngagementNumber = "0000123456";
        member1.EmployeeId = "13893";
        var member2 = new EngagementTeamMember();  
        member2.EngagementNumber = "0000123456";
        member2.EmployeeId = "13893"; 
        Console.WriteLine(p1.Equals(p2));  // false
        Console.WriteLine(p1 == p2);  // false
        Console.ReadKey();  
    }  
}  

判斷相等的方式, 就與想像中的不同 `Distinct()`, `Except()`, `Intersect()`, `Union()` 會出現想像之外的結果, 也就不奇怪了

#### Reference url
- [== Operator and Reference Types in C#](https://www.c-sharpcorner.com/article/story-of-equality-in-net-part-five/)

## 如何用定義 Reference types 相等

1. 繼承 `IEquatable<T>` Interface
2. 使用 建構子 `constructor` 設定 屬性 `properties`
3. 並實作 `Equals()` Method
4. 覆寫 Object.GetHashCode，以便有實值相等的兩個物件產生相同的雜湊碼。

### 繼承 `IEquatable<T>`

 protected class EngagementTeamMember : IEquatable<EngagementTeamMember>
 {
     public string EngagementNumber { get; }
     public string EmployeeId { get; }

     public EngagementTeamMember(string engagementNumber, string employeeId) //Set the properties in the constructor
     {
         EngagementNumber = engagementNumber;
         EmployeeId = employeeId;
     }

     public bool Equals(EngagementTeamMember other) //providing a type-specific Equals method.
     {
         return other != null && (EngagementNumber == other.EngagementNumber && EmployeeId == other.EmployeeId);
     }

     public override int GetHashCode() //Override Object.GetHashCode so that two objects that have value equality produce the same hash code.
     {
         return new { EmployeeId, EngagementNumber }.GetHashCode();
     }
 }

#### Reference url
- [如何：定義類型的實值相等 (C# 程式設計指南)](https://docs.microsoft.com/zh-tw/dotnet/csharp/programming-guide/statements-expressions-operators/how-to-define-value-equality-for-a-type)

這樣修改完在``相等的判斷就沒有會依照想像的方式回傳結果

`Distinct()`, `Except()`, `Intersect()`, `Union()` 也可以正常運作了

## But 遇到不同的情境, 需要改變判斷相等的條件怎麼辦

上面已經定義了，如果兩個案件 `EngagementNumber` 和 `EmployeeId` 相同, 才會認為相同

今天要是有一個特別的需求，在這個情境下只要 `EngagementNumber` 相等, 兩個物件就算相等

### 使用 `IEqualityComparer<T>` 在各別情境做比較

新增一個 `Class` `EngagementTeamMemberEngagementNumberEqualityComparer` 繼承 `IEqualityComparer<EngagementTeamMember>`

在做 `LINQ` 做 `Distinct()`, `Except()`, `Intersect()`, `Union()` 使用多載的方式, 把 `IEqualityComparer<T>` 傳入

- `Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>)`
- `Distinct<TSource>(IEnumerable<TSource>, IEqualityComparer<TSource>)`
- `Union<TSource>(IEnumerable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>)`
- `Intersect<TSource>(IEnumerable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>)`

就會用 `IEqualityComparer<T>` 定義的方式判斷

 protected class EngagementTeamMember : IEquatable<EngagementTeamMember>
 {
     public string EngagementNumber { get; }
     public string EmployeeId { get; }

     public EngagementTeamMember(string engagementNumber, string employeeId) //Set the properties in the constructor
     {
         EngagementNumber = engagementNumber;
         EmployeeId = employeeId;
     }

     public bool Equals(EngagementTeamMember other) //providing a type-specific Equals method.
     {
         return other != null && (EngagementNumber == other.EngagementNumber && EmployeeId == other.EmployeeId);
     }

     public override int GetHashCode() //Override Object.GetHashCode so that two objects that have value equality produce the same hash code.
     {
         return new { EmployeeId, EngagementNumber }.GetHashCode();
     }
 }
   
 protected class EngagementTeamMemberEngagementNumberEqualityComparer : IEqualityComparer<EngagementTeamMember>
 {
     public bool Equals(EngagementTeamMember x, EngagementTeamMember y)
     {
         return y != null && x != null && x.EngagementNumber == y.EngagementNumber;
     }
   
     public int GetHashCode(EngagementTeamMember obj)
     {
         return obj.EngagementNumber.GetHashCode();
     }
 }

 /// <summary>
 /// Sets the engagement team members.
 /// </summary>
 public partial class DeclarationMainDataService
 {
     private void SetEngagementTeamMembers()
     {
         var teamMemberEqualityComparer = new EngagementTeamMemberEngagementNumberEqualityComparer();
         var engagementTeamMembers = GetEngagementTeamMember(teamMemberEqualityComparer)
             .ToList();
         var engagementMemberByTimeSheet = GetEngagementMemberByTimeSheet(teamMemberEqualityComparer)
             .ToList();
         var allTeamMembers = engagementTeamMembers
             .Union(engagementMemberByTimeSheet, teamMemberEqualityComparer);
         var existedEngagementTeamMembers = GetExistedEngagementTeamMembers()
             .Distinct(teamMemberEqualityComparer);
         var allNeedAddTeamMembers = allTeamMembers
             .Except(existedEngagementTeamMembers, teamMemberEqualityComparer)
             .ToList();
         var teamMembersByTimeSheet = allNeedAddTeamMembers
             .Except(engagementTeamMembers, teamMemberEqualityComparer)
             .ToList();
         var needAddTeamMembers = allNeedAddTeamMembers
             .Except(teamMembersByTimeSheet, teamMemberEqualityComparer)
             .ToList();
         EngagementTeamMembers = needAddTeamMembers
             .GroupBy(c => c.EngagementNumber)
             .ToDictionary(c => c.Key, h => h.Distinct().ToList());
         EngagementTimeSheetUsers = teamMembersByTimeSheet
             .GroupBy(c => c.EngagementNumber)
             .ToDictionary(c => c.Key, h => h.Distinct().ToList());
     }
     
     /// <summary>
     /// Gets the engagement team member.
     /// </summary>
     /// <param name="teamMemberEqualityComparer"></param>
     /// <returns>IQueryable&lt;EngagementTeamMember&gt;.</returns>
     private IEnumerable<EngagementTeamMember> GetEngagementTeamMember(EngagementTeamMemberEqualityComparer teamMemberEqualityComparer)
     {
         var engagementNumbers = EntityNumberKey.Select(c => c.EngagementNumber).ToList();
         var tempEngagementTeamMembers = new List<EngagementTeamMember>();
         var resultTeamMember = GetEngagementTeamMembersFromTeamMemberTable(engagementNumbers);
         var resultEngagementTeamMembers = GetEngagementHeaderMemberPersonnelNumber(engagementNumbers);
         tempEngagementTeamMembers.AddRange(resultTeamMember);
         tempEngagementTeamMembers.AddRange(resultEngagementTeamMembers);
         return tempEngagementTeamMembers.Distinct(teamMemberEqualityComparer);
     }
 }

#### Reference url
- [MSDN Enumerable.Distinct Method](https://docs.microsoft.com/zh-tw/dotnet/api/system.linq.enumerable.distinct?view=netframework-4.8)
- [MSDN Enumerable.Except Method](https://docs.microsoft.com/zh-tw/dotnet/api/system.linq.enumerable.except?view=netframework-4.8)
- [MSDN Enumerable.Union Method](https://docs.microsoft.com/zh-tw/dotnet/api/system.linq.enumerable.union?view=netframework-4.8)
- [MSDN Enumerable.Intersect Method](https://docs.microsoft.com/zh-tw/dotnet/api/system.linq.enumerable.intersect?view=netframework-4.8)